The present invention relates to a ceramic sheathed-element glow plug for diesel engines.
Glow plugs having external ceramic heaters are discussed, for example, in German Published Patent Application No. 40 28 859. Furthermore, German Published Patent Application No. 29 37 884 discusses metallic sheathed-element glow plugs in which the metallic glow filament is welded to a thermoelement. During the operation of the sheathed-element glow plug, the temperature of the respective cylinder can be measured here by measuring the heat-generated voltage. However, it is believed that no metallic glow filament is present in a sheathed-element glow plug having a ceramic heating element.
Furthermore, German Published Patent Application No. 198 44 347 discusses a sheathed-element glow plug having a connecting element which is electrically connected to the glow element via a contacting element. The contacting element is shown as a spring in FIG. 1.
The exemplary ceramic sheathed-element glow plug according to the present invention is believed that the advantage that the temperature of the glow element is measurable. In a sheathed-element glow plug, the temperature of the glow element can be measured directly in a selected area on the outside of the glow element without additional hardware. The temperature is measured in a selected volume that is small in comparison with the volume of the entire glow element, so that the error occurring due to temperature distribution over a large volume can be reduced in determining the temperature. Also, in the exemplary sheathed-element glow plug according to the present invention, the heating power can be concentrated to a selected area of the glow element without modifying the cross-section of the conductive layer, so that the surface area remains constant in the area in which the heating power is to be concentrated and thus also the interaction area is kept constant. It is also believed that such a ceramic sheathed-element glow plug can be manufactured in a cost-effective manner.
In particular, by selecting the appropriate ceramic materials used for the different areas of the sheathed-element glow plug, it is believed that it is ensured that the mechanical stability of the heater is not impaired. By having the measured temperature values processed by a controller, the temperature can be regulated in the selected area of the glow element. Also, the exemplary sheathed-element glow plug according to the present invention may be used in a passive mode as a temperature sensor after it has performed its heating function. Thus it can be determined whether the combustion process is taking place correctly in the respective cylinder. It is believed that the parameters relevant to the combustion can be influenced on the basis of this information.
The exemplary ceramic sheathed-element glow plug according to the present invention is believed to have the advantage that, due to the greater conductive cross-section, higher currents can be transmitted without causing heat damage to the material of the contacting element. The large surface area of the contacting material is also believed to be advantageous because it provides good heat conductivity. It is believed that the elastic spring component ensures that thermal displacements of surrounding components due to different thermal expansion coefficients can be compensated.
The contacting element may be made of graphite or a conductive ceramic powder, since these materials are corrosion-resistant. Also, only a predominant portion of the material may be made of graphite or conductive ceramic or metal powder, thus saving on expensive materials while achieving approximately the same characteristics. If the sheathed-element glow plug having a contacting element according to the exemplary embodiment of the present invention is manufactured as described below, it is believed that this provides an arrangement of the components located in the glow element housing that prevents short-circuits. In addition, it is believed to be ensured that the components are pressed together so that they do not come loose or burst due to the excessive reactive force of elastic elements (e.g., of the contacting element).